62 EXPLORING EXPEDITION FROM SANTA FI3 



The existence of great volcanos, like San Francisco Mountain and San Mateo, in 

 full blast, man)- hundred miles from the sea, is a powerful argument against the theory 

 which restricts all volcanos to the vicinity of large bodies of water. 



SIERRA TUCANE (NAVAJO MOUNTAIN). 



This mountain is situated near the junction of the San Juan and Colorado 

 Rivers, and was, therefore, not reached by either of the expeditions with which I 

 have been connected. As seen from a distance, it seems to be a mountain of very 

 considerable elevation, either solitary or associated witli a few subordinate peaks, 

 forming a group of limited extent. What its structure is we can, of course, only con- 

 jecture, but from its isolated character we may be quite sure that it is composed of 

 erupted material; and it should probably be grouped with the North Side Mountains, 

 the Sierra Abajo, Sierra La Sal, La Late, &c, which are scattered like islands over 

 the surface of the plateau. The description of these latter groups, as well as that of 

 the Rocky Mountain ranges lying on or near our trail, will be given in the succeeding 

 chapters devoted to the geology of our route. 



TIIE COLORADO PLATEAU. 



In the preceding pages, where I have referred to the area under consideration, 

 I have used indiscriminately the terms plateau and basin; the one having reference 

 to the prevailing character of the surface, the other to the geological structure. Both 

 terms are sufficiently appropriate; for, when viewed as a whole, it has a distinctly 

 plateau character, formed by a series of table-lands, which rise step by step from the 

 Carboniferous limestone to the summit of the Cretaceous formation, and from the broad 

 valleys of erosion, in which its canons are scored, to the summits of the divides between 

 its draining streams. In the geological portion of the report of the Colorado Expedition 

 will be found both a general and a detailed description of the topography and structure 

 of the central and southern portions of the Colorado Plateau. The observations made 

 on our recent explorations enable me to complete, in a great degree, the sketch, much 

 of which was then only given in outline ; to make important additions to the informa- 

 tion then gained of both the geographical and vertical extension of the sedimentary 

 strata which are there represented, and, by satisfactory proofs, confirm views before 

 only conjectural. 



Adding to the facts then given those more recently observed, we find that the 

 generalities of the structure of the Colorado plateau or basin are as follows : 



On both the eastern and western sides, Carboniferous strata, the equivalents of the 

 Coal-Measures in age, are seen resting on the granitic axes of the bounding mountains, 

 dipping at first rapidly, then more gradually, toward the center of the basin or 

 •trough. On the eastern side the Carboniferous strata are exposed only in a narrow 

 belt along the -mountain-sides, almost immediately passing beneath the Triassic and 

 Cretaceous strata, which there have an aggregate thickness of 5,000 feet or more. On 

 the western side, on the contrary, the first easterly dip of the Carboniferous rocks is 

 succeeded by a gentle rise toward the east ; a broad arch being formed, of which the 



